The present invention relates primarily to a method for polishing bladed disks and comprising an airflow channel for a turbomachine, more particularly a method for polishing centrifugal impellers for a turbomachine compressor and single-piece bladed disks, and a polishing device for implementing such a method.
Turbomachines traditionally comprise a compressor, a compression chamber and a turbine.
The compressor is intended to increase the pressure of the atmospheric air, the combustion chamber mixes the air that is compressed by the compressor with fuel and burns the mixture, and the turbine, placed in the discharged flow, is driven by that flow of very hot air. It serves to drive the compressor via the axis of the turbomachine.
The compressor comprises rotors, said rotors comprising bladed disks, some of which are called centrifugal impellers, and stators. A centrifugal compressor impeller, hereinafter called impeller, comprises a substantially tapered body and blades distributed over the entire surface of the body.
These blades delimit, two by two with the tapered body, an air flow channel in the form of a helix portion.
A centrifugal compressor impeller therefore has a complex shape.
This impeller is, for certain applications, cut directly in the mass, for example in a block of titanium or nickel alloy. Such an impeller can also be obtained by casting, by rapid prototyping or electrochemically.
Moreover, due to the aerodynamic function the centrifugal compressor impellers must perform, the surface condition of the impeller, more particularly the surface of the tapered body forming the bottom of the channel along which the air flows, and that of the blades, is very important and very particular care is given to the production thereof.
To meet the aerodynamic conditions of the air flowing on the impeller, the surface parameter Ra must not exceed 0.6 μm (Ra is a statistical value and corresponds to the mean arithmetic deviation relative to the center line; Rt is the maximum height of the peaks). However, this roughness value cannot be obtained directly by machining, casting or another technique for making the impeller. A polishing step is therefore necessary in order to achieve the required surface quality.
There are several techniques for polishing such parts.
The polishing can be done manually using abrasive belts. This technique has the advantage of making it possible to polish pieces with complex shapes. However, this polishing takes a very long time, and is therefore costly in terms of labor. Moreover, its quality depends entirely on the operator performing the polishing.
Machines, like those described in U.S. Pat. No. 2,547,056, can be used, but they are very complex structures and do not make it possible to polish parts with complex shapes.
Polishing can also be done using abrasive particles, as described in document JP 57211469. This technique provides for mounting a cover on the impeller so as to enclose the active zone of the impeller comprising the blades in a closed space and placing abrasive particles in that volume, then making the impeller rotate around its horizontal axis. The rotation and gravity force cause the particles to move on the surface to be polished. When the required surface state is reached, the rotation of the impeller is interrupted, the cover and the particles are removed. With this technique, there is a risk of not achieving the desired surface parameter Ra due to stagnation of the abrasive particles in the zones in question.
It is therefore one aim of the present invention to propose a method for polishing centrifugal impellers, and more generally bladed turbomachine parts, that is simple, adapted to all types of impellers regardless of the complexity of their shapes, and offering a particularly high-performance surface state for the flow of air.
It is also an aim of the present invention to propose a device for polishing bladed disks that is simple and robust.